In the diagram below assumed to be a feeder line that is, power flows only in one direction on the transmission line (from left to right) the three protection zones extending from bus A are being shown each protection zone having its own element (relay). Appropriate three protection zones extend from bus B towards bus C and from bus C further to the next bus. So, on every bus there are three elements (relays) for the three protection zones.

A1 - First element (relay) on bus A overviewing the first zone extending from bus A to bus B.

A2 – Second element (relay) on bus A overviewing the second zone extending from bus A beyond bus B.

A3 – Third element (relay) on bus A overviewing the third zone extending from bus A beyond bus C.

B1 – First element (relay) on bus B overviewing the first zone extending from bus B to bus C.

B2 – Second element (relay) on bus B overviewing the second zone extending from bus B beyond bus C.

B3 – Third element (relay) on bus B overviewing the third zone extending from bus B beyond bus D.

1. If a fault occurs in the middle of transmission line1 between bus A and bus B which relays will be activated and which relay will trip first? Which circuit breaker will open? (assuming all relays function reliably)
2. Now assume relay A1 is defect and the same fault occurs as before, which relays will be activated and which relay will trip first?
3. If a fault occurs in the first half of the transmission line2 connecting bus B and bus C and all relays function properly which relays will be activated and which relay will trip first?
4. Describe now a scenario during which a fault occurs on in the first half of the transmission line2 connecting bus B and bus C but the circuit breaker at bus A will trip and not the one at bus B.
5. In which of the four above questions do we have local back-up and remote back-up?
    

BUS ELEMENT I ELEMENT 2 ELEMENT 3 LINE 1 ZONE 1 BUS B LINE 2 ZONE 2 BUS LINE ZONE 3